The present invention relates generally to electronic dimming ballasts for gas discharge lamps.
More particularly, this invention pertains to a software controlled electronic dimming ballast.
Electronic dimming ballasts are well known in the art. These ballasts are typically designed to be connected to and receive power from an alternating current (AC) power source and to be connected to and supply AC power to a lamp load including one or more gas discharge lamps. To facilitate this function, prior art ballasts usually include an alternating current/direct current (AC/DC) converter circuit 14 and an inverter circuit 18. The AC/DC converter circuit 14 converts low frequency AC voltage supplied by the AC power source into DC voltage and the inverter circuit 18 converts the DC voltage supplied by the AC/DC converter circuit 14 into high frequency AC voltage. In some applications, prior art ballasts may also include an electromagnetic interference (EMI) filter circuit, which is used to filter out noise in the ballast, and a power factor control circuit that is used to ensure that the power factor associated with the ballast stays at a desired level.
As the name suggests, electronic dimming ballasts are also designed so that they can be used to dim the light output of the lamp load connected to one of these ballasts. To facilitate this function, electronic dimming ballasts usually include some type of dimming control circuit that can be used to decrease and increase the light output of the lamp load. The dimming control circuit usually accomplishes this function by simply decreasing and increasing the power supplied by the inverter circuit 18 to the lamp load.
An example of a prior art electronic dimming ballast manufactured and sold by the assignee of the present invention, Universal Lighting Technologies, is shown in FIG. 1. This electronic dimming ballast is designed to be connected to and receive low frequency AC voltage from an AC power source and to supply high frequency AC voltage to a lamp including one or more gas discharge lamps. The ballast includes an EMI filter circuit, an AC/DC converter circuit, a PFC circuit, an inverter circuit, and a dimming control circuit.
The dimming control circuit includes a dimming control signal conditioning circuit, a microcontroller circuit, a pulse width modulator (PWM) circuit, and a lamp current sensing circuit. The dimming control signal conditioning circuit is used to receive a dimming control signal from an appropriate dimming control device and to generate a conditioned dimming control signal that can be applied to the microcontroller circuit. The microcontroller circuit is designed to generate a microcontroller dimming control signal based on the conditioned dimming control signal and to supply that signal to the PWM circuit. The lamp current sensing circuit is designed to receive a lamp current signal from the lamp load, to generate a lamp voltage signal based on that current signal, and to supply that voltage signal to the PWM circuit.
The PWM circuit uses the microcontroller dimming control signal and the lamp voltage signal to generate and supply a pulse width modulated inverter dimming control signal to the inverter circuit in the ballast. More specifically, the PWM circuit generates an error signal by comparing the microcontroller dimming control signal and the lamp voltage signal using a differential amplifier and uses that error signal to generate the appropriate pulse width modulated inverter dimming control signal. By varying the pulse width of the inverter dimming control signal, the dimming control circuit can vary the power supplied by the inverter circuit to the lamp and, as a result, can control the lamp load light output.
Although the electronic dimming ballast shown in FIG. 1 does allow one to control the dimming level of the lamp load, it has several disadvantages. First, the dimming control circuit in this ballast requires a relatively high number of electronic components, i.e., the dimming control signal conditioning circuit, the microcontroller circuit, the PWM circuit, and the lamp current sensing circuit, and takes up a large amount of space in the ballast. This increases the size of the ballast and makes it undesirable in applications where available space is limited. Second, the high number of electronic components increases the overall cost of the ballast and makes it unsuitable for certain applications.
What is needed, then, is an electronic dimming ballast that requires fewer components, is smaller, and is less expensive than the electronic dimming ballast discussed above, as well as other prior art electronic dimming ballasts suffering from similar problems.